System &amp; method for controlling camshaft timing, air/fuel ratio, and throttle position in an automotive internal combustion engine

ABSTRACT

A system for controlling camshaft timing, air/fuel ratio and electronic throttle position in an automotive internal combustion engine uses a controller for operating a camshaft phaser, electronic throttle positioner and fuel injectors. The controller determines camshaft timing, steady-state electronic throttle position, steady-state fuel supply, and compensatory transient electronic throttle position, and transient fuel supply such that an engine operating with the present system has the torque output characteristics matching a conventional engine having fixed camshaft timing, but with lower fuel consumption and lower exhaust emissions than a conventional engine.

This application is a divisional of Ser. No. 09/005,571, filed Jan. 12,1998, now U.S. Pat. No. 6,006,725.

FIELD OF THE INVENTION

The present invention relates to a system and method for simultaneouslycontrolling transient camshaft timing, air/fuel ratio, and electronicthrottle position in an internal combustion engine.

BACKGROUND OF THE INVENTION

Lean-burn operation of spark-ignited internal combustion engines isdesirable because it improves fuel economy. By combining lean-burn andvariable cam timing (VCT) technologies in port fuel injected engines,improvement in fuel economy of about 8 to 10% can be achieved. Moreover,available data suggest that the feedgas emissions of a lean-burn VCTengine are also improved. As used herein, the term “feedgas” means theexhaust gas leaving the engine prior to any aftertreatment. And, theterm VCT refers to engine cylinder valve timing control of either intakeand exhaust valves or only exhaust valves.

Additional improvement in efficiency is possible by operating a directinjection spark-ignited (DISI) engine in a very lean stratified-chargemode. The present invention addresses the problem of scheduling camshafttiming, air/fuel ratio, and electronic throttle position for leadoperation of both port-injection and DISI engines in order to achieveoptimum performance in terms of fuel efficiency and emissions as well asthe driveability or torque response of a conventional engine.

For the purposes of this specification, it is assumed that the engine isequipped with an electronic throttle control (ETC) in which the vehicledriver merely operates a potentiometer, with the actual throttle openingbeing determined by the engine's electronic controller. Other sensorsand actuators used with conventional electronically controlled enginesmay be employed with a system and method according to the presentinvention. Performance of the present system during lean operation maybenefit from a universal exhaust gas oxygen (UEGO) sensor used insteadof or in conjunction with a heated exhaust gas oxygen(HEGO) sensor.

The additional degrees of freedom available in a lean-burn VCT enginemake the scheduling of camshaft timing, air/fuel ratio, and electronicthrottle position difficult. The method proposed in this specificationis structured to decouple driveability issues from the steady satescheduling of the ETC, camshaft timing and air/fuel ratio. The optimalsteady state schedules are obtained using the engine data of fuelconsumption and HC, CO, and NO_(x) emissions at different cam andair-fuel values with engine speed and braking torque held constant.Demanded torque at a given engine speed can be achieved by manydifferent combinations of throttle position, camshaft timing, andair/fuel ratio. The present system and method assures that, at a giventorque demand, the steady state values of camshaft timing and air/fuelratio are optimal.

Transient operation of the ETC, the air/fuel ratio and camshaft timingmust be carefully managed in order to achieve torque response resemblinga conventional engine. Because the ETC (as an actuator) and fuelinjectors are much faster than the cam timing actuator, the followingsequence is employed for scheduling and dynamic transient compensation:(a) cam timing command is as prescribed by optimal steady stateschedules; (b) the ETC command contains a component which is used tocompensate for the cylinder air-charge variation due to cam timingtransients; and (c) the air/fuel ratio command contains a dynamiccomponent that matches the manifold filling dynamics to avoid largetorque excursions and driveability problems. In general, when valvetiming is moved from a more retarded position to a more advancedposition, the ETC must be placed in a more closed position; conversely,when valve timing is moved from a more advanced position to a moreretarded position, the ETC must be placed in a more open position.

One distinct feature of the proposed method is that the air/fuelscheduling into the lead region is air-driven not fuel-driven. Thismakes the task of simulating the driveability of a conventional enginemuch easier because engine output torque is much more sensitive to fuelchanges at constant air than to air changes at constant fuel. Forexample, for a fixed flow of fuel, changing the air flow from 20:1 leanto stoichiometric changes the engine torque by about 6% to 8% as thisonly changes the efficiency of the engine. On the other hand, for afixed air flow, changing the fuel flow from 20:1 lean to stoichiometricchanges the torque by more than 30%.

To meet legislated tailpipe emission requirements, lean-burn enginesmust be equipped with a “lean NO_(x) trap” (LNT) to reduce the exhaustconcentration of the oxides of nitrogen (NO_(x)). The LNT requiresperiodic purging, which is accomplished by operating the engine ateither exact stoichiometry or at a rich air/fuel ratio for a period oftime. Changing the amount of fuel from lean to rich operation causes anincrease in torque which is not demanded by the driver, resulting indriveability problems. The present air-driven method of operation avoidsthis problem and allows purging of an LNT without causing torquevariation.

During stratified operation of DISI engines the problem of fuel-drivenair-fuel control is even more pronounced and the benefits of the presentair-driven scheduling is more significant.

SUMMARY OF THE INVENTION

According to the present invention, a system for controlling thecamshaft timing, air/fuel ratio, and electronic throttle position in anautomotive internal combustion engine includes a camshaft phaser forcontrolling the timing advance of a camshaft for operating cylinderintake and exhaust valves of the engine, a throttle position sensor forsensing the position of a manually operable accelerator and forproducing an accelerator position signal, and an engine speed sensor forsensing engine speed and for producing an engine speed signal. Thepresent system also includes an electronic throttle positioner forsetting an intake air throttle at a commanded position, a plurality offuel injectors for supplying fuel to the engine, and a controller foroperating the camshaft phaser, the electronic throttle positioner, andthe fuel injectors, with the controller receiving the outputs of theaccelerator position and engine speed sensors, and with the controllerdetermining camshaft timing advance, steady-state and transientelectronic throttle position, and fuel supply.

According to another aspect of the present invention, the controllerdetermines an ETC setting appropriate to achieve a rich air/fuel ratiosuitable for purging a lean NO_(x) trap based upon a quantity of fuelsuitable for operating the engine at approximately a stoichiometricair/fuel ratio, but with excess air sufficient to cause enleanment ofthe air and fuel mixture.

The engine controller of the present system operates the engine withimproved fuel economy by operating the fuel injectors to provide aquantity of fuel suitable for operation at approximately astoichiometric air/fuel ratio, but with the camshaft phaser and theelectronic throttle being operated so as to provide an air charge havingsufficient mass so as to operate with a lean air/fuel ratio. This isessential to a fuel-driven operating system, rather than the air-drivensystems found in the prior art.

The engine controller determines the transient electronic throttleposition as a function of at least the time rate of change of thecamshaft timing, and preferably, the instantaneous pressure within theengine's inlet manifold.

According to another aspect of the present invention, a method forcontrolling the camshaft timing, air/fuel ration, and electronicthrottle position in an automotive internal combustion engine comprisesthe steps of determining camshaft timing advance value for a camshaftwhich operates cylinder intake and exhaust valves of the engine,determining a steady-state position for an electronic air throttle,determining a steady-state fuel supply rate, and determining transientvalues for electronic air throttle position, and fuel supply rateappropriate to migrate to a desired rich or lean air/fuel ratio whileallowing engine torque output to closely approximate the torque outputof the engine without camshaft timing control.

The camshaft timing advance and steady-state air/fuel ratio arepreferably based upon a sensed operating position of a manually operableaccelerator, as well as upon sensed engine speed.

The present method may further comprise the step of determining a richair/fuel ratio suitable for purging a lean NO_(x) trap based upon aquantity of fuel suitable for operating the engine at approximately astoichiometric air/fuel ratio, but with a reduction in air sufficient tocause enrichment of the air and fuel mixture. And, the present methodmay further include the step of determining a lean air/fuel ratio foroperating the engine with increased fuel economy, followed by operationof fuel injectors so as to provide a quantity of fuel suitable foroperation at approximately a stoichiometric air/fuel ratio, but withsaid camshaft phaser and said electronic throttle being operated so asto provide a sufficient air charge so as to operate with a lean air/fuelratio.

According to another aspect of the present invention, transient air/fuelratio is determined so as to track and follow the filling and emptyingof the intake manifold, with the result that fluctuations in outputtorque are minimized.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of an engine having a controlsystem according to the present invention.

FIG. 2 is a plot of electronic throttle position as a function ofaccelerator position.

FIG. 3a is a plot of camshaft position as a function of engine speed andelectronic throttle position.

FIG. 3b is a plot of air/fuel ratio as a function of engine speed andelectronic throttle position.

FIG. 4 is a plot of air charge as a function of engine speed andelectronic throttle position.

FIG. 5 is a plot of engine air flow as a function of the pressure ratioacross the throttle.

FIG. 6 is a plot of engine air flow as a function of throttle engine.

FIGS. 7A and 7B are plots of engine airflow as a function of enginespeed and camshaft position.

FIG. 8 is a plot of intake manifold reference pressure as a function ofthrottle angle and engine speed.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In order to operate an engine according to the present invention, it isnecessary to know throttle position, as governed by an ETC, camshaftoperating position (also described as camshaft timing), and fuelinjector pulse width. Of course, knowing fuel injector pulse width andthe operating characteristics of the camshaft and electronic throttle,the air/fuel ratio can be set to the desired rich, lean, orstoichiometric point. In this manner the engine may be operated toachieve the best emissions and fuel economy with a torque response whichis the same as a conventional engine operating at the stoichiometricair/fuel ratio and fixed camshaft timing.

FIG. 1 illustrates an engine having a system according to the presentinvention. Engine 10 is provided with fuel by means of fuel injectors 12which are operated by engine controller 14. Engine controller 14 is ofthe conventional variety known to those skilled in the art and assuggested by this disclosure. Controller 14 also operates camshaftphaser 16 which controls the timing advance of a camshaft which operatesthe cylinder valves of engine 10.

Controller 14 receives an engine speed signal from speed sensor 18 aswell as a variety of other engine operating parameters from sensor 20,which include such sensors as an intake manifold pressure sensor andother sensors known to those skilled in the art and suggested by thisdisclosure. A system according to the present invention further utilizeselectronic throttle positioner 22 which in effect provides adrive-by-wire because the intake air throttle (not shown) is set at aposition commanded solely by controller 14; throttle position sensor 24merely senses or measures the position of a manually operableaccelerator and produces an accelerator position signal. In other words,the vehicle operator has no direct link with the air throttle admittingair to the engine's intake manifold.

FIG. 2 illustrates a plot of accelerator position AP versus electronicthrottle command θ_(c). This is a calibratable function which will giveprogressivity to the vehicle driver's accelerator command according tothe dictates of an engineer doing development work on a vehicle havingan engine and system according to the present invention. In other words,the greater the slope of the plot, the more aggressively electronicthrottle positioner 22 will open the air throttle and in response thedriver's input. Simply state, θ_(c) is a measure of the torque responsedemanded by the driver because the more aggressively the driverdepresses the accelerator pedal, the greater the driver's expectation ofengine response.

Having received a value θ_(c) from the plot of FIG. 2, which can bemerely a lookup table within controller 14, the engine controller movesto FIGS. 3a and 3 b, which is a three-dimensional plot, again in theform of a lookup table having as independent variables θ_(c) from FIG.2, and engine speed, N. The plots of FIGS. 3a and 3 b, which aredetermined from engine mapping data, give camshaft position Γ^(ref) aswell as air/fuel ration, α^(ref). The camshaft timing and air/fuel ratioselected from three-dimensional lookup tables by controller 14 at thisstep provide the camshaft timing and air/fuel required to achieveoptimal emissions and fuel economy at the driver's demanded torque. Thisis the torque generated at the given accelerator pedal position andengine speed by an engine operating with fixed camshaft timing and astoichiometric air/fuel ratio.

It is noted here that the camshaft position and air/fuel ratio vary toprovide the best emission control capability. These are optimal steadystate values dependent on engine speed and torque.

Knowing the steady-state camshaft position and air/fuel ratio, it isstill necessary to determine the required throttle position and fuelinjector pulse width.

To calculate the fuel charge required at stoichiometry, one need merelytake the cylinder air charge and divide by 14.64, which is thechemically correct air/fuel ratio for a typical gasoline motor fuel.Thus, having determined the desired fuel charge, it is necessary tocalculate the throttle angle required to achieve the lean air/fuel ratioα^(ref) given the fuel flow previously calculated. As explained above,lean operation is desired for reasons of fuel economy and emissioncontrol. Also, we must determine an additional dynamic correction ofthrottle position to avoid torque disturbances due to moving camshafttiming per the schedule calculated for Γ^(ref). The required cylinderairflow is calculated as:

φ_(cyl) ^(o)=(desired fuel charge)×(α^(ref)).

The values of φ_(cyl) ^(o) and N are sued to determine from enginemapping data throttle angle θ^(o) to provide the desired air flow forlean operation. For the purpose of purging an LNT, the engine mustoperate with a rich air/fuel ratio, α^(rich). In the same manner θ^(o)was determined, a new air flow for rich operation can be determined,along with a required ETC setting for rich operating, θ^(r). Thus, therequired cam position, Γ^(ref) and throttle positions θ^(o) and θ^(r)are stored as functions θ_(c) and N in lookup tables in the memory ofcontroller 14. These lookup tables are used in real time to assure lowemissions, good fuel economy, and the driveablility of a conventionalengine.

To assure that the engine's torque response is close to that of aconventional engine, the steady state schedules described above are notused directly for controlling cam timing, throttle position, andair/fuel ratio. Instead, controller 14 adds a dynamic correction to ETCposition and air/fuel ratio command to avoid engine output torquedisturbances. The sequence of action of controller 14 may be summarizedas follows.

1. The reference camshaft timing position, Γ^(ref) obtained from thelookup table illustrated in FIG. 3a is used directly by camshaft phaser16 to control cam timing. In other words, the camshaft timing command isnot filtered. This is true because the response time of camshaft phaser16 generally slower than the response times of the ETC or fuelinjectors.

2. An additional throttle angle, θ*, needed to compensate for torquedisturbance caused by camshaft movement, is computed and added to θ^(o),the lean operation throttle setting, obtained from the lookup tableillustrated in FIG. 4. The sum of θ^(o) and θ* is then used to commandthe ETC to a desired position.

3. Steady-state air/fuel ratio command, α^(ref), is modified to accountfor the dynamics the intake manifold. The modified value of α^(ref) isthen sued to determine the amount of fuel to be delivered by fuelinjectors 12.

4. For urging of an LNT, steps 2 and 3 above are repeated for θ^(r).Steps 1 and 4 are straightforward to implement; steps 2 and 3 will beexplained below.

Controller 14 must calculate an additional throttle angle that whenadded to the angle calculated above compensates for the torquedisturbance caused by moving camshaft to the desired position Γ^(ref).This is a dynamic correction which will only be applied while thecamshaft phaser 16 is moving the camshaft to a new position. To makethis correction, controller 14 needs to know the mass airflow throughthe throttle body and into the intake manifold. This is a well knownfunction of pressure ratio across the throttle valve and upstreamtemperature and pressure. Graphically, this may be represented as two ormore functions g₁, g₂. FIGS. 5 and 6 illustrate these functions. FIG. 5is a plot of engine airflow at standard temperature and pressure as afunction of pressure ratio across the throttle, Pm/Pa.

FIG. 6 is a plot of engine airflow at standard temperature and pressureas a function of throttle angle. The flow across the throttle, φ_(θ),equals g₁×g₂. The flow rate of air from the intake manifold into theengine's cylinders can be represented by an additional functioncomprising two parameters which are functions of engine speed andcamshaft timing position, plus intake manifold pressure P_(m), which isa measured quantity.

FIGS. 7A and 7B illustrate parameters α₁ and α₂ which are functions ofcamshaft position and engine speed. The values of α₁ and α₂, which arereadily available from engine mapping, are stored in lookup tableswithin controller 14. Flow into the cylinders is calculated as:

φ_(cyl)=α₁ P _(m)+α₂.

Prior to making the final throttle correction, the reference manifoldpressure, P_(mref) needs to be known. This is intake manifold pressurecorresponding to a stoichiometric fixed cam engine operating at a givenengine speed. This may be calculated from the perfect gas law ortabulated.

FIG. 8 illustrates intake manifold pressure P_(mref) as a function ofengine speed N and θ^(o). Controller 14 now determines a throttlecorrection by solving the following equation. Then, the transientthrottle correction is θ=θ^(o)+θ*.

θ*=(g ₂)⁻¹{[(∂α₁∂Γ_(cam)) P _(m)+(∂α₂/∂Γ_(cam))]·[dΓ _(cam) /dt]/[k _(m)g ₁(P _(m))α₁ ]+[g ₁(P _(mref))/g ₁(P _(m))]g ₂(θ^(o))}−θ^(o)

At this point, controller 14 has determined throttle position asθ=θ^(o)+θ* for the ETC.

For step 3 above, an additional calculation is required, that is,calculation of the fuel injector pulse width. Because fuel charge can bechanged faster than air, a change in fuel charge will cause anundesirable air/fuel transient unless the fuel command is shaped untilthe air flow catches up. This is accomplished by filtering the air/fuelratio command to account for the lag. The essential differentialequation is given as shown below:

de/dt=K _(m) [g ₁(P _(mref))g ₂(θ^(o))−g ₁(P _(mref) −e)g₂(θ_(c))−α₁(N,O)e]

Here, α₁ is evaluated without any camshaft advance. Then, the correctionfactor applied to the air flow command is thus Δφ_(cyl)=α₁(N,θ)e andΓ_(AF) which is the commanded air/fuel ratio compensated for manifoldfilling dynamics is given by the expression:

Γ_(AF)=14.64[1+Δφ_(cyl)/(φcyl−Δφ _(cyl))]

In summary, controller 14 calculates Γ^(ref), which is camshaftposition, Γ_(AF) which is the air/fuel ratio accounting for manifolddynamics, and θ*, the throttle command accounting for camshaft phaserdynamics. During transient operation, the camshaft position, the ETCposition, and the air/fuel ratio all change continuously.

While the invention has been shown and described in its preferredembodiments, it will be clear to those skilled in the arts to which itpertains that many changes and modifications may be made thereto withoutdeparting from the scope of the invention.

What is claimed is:
 1. A method for controlling the camshaft timing, air/fuel ratio, and electronic throttle position in an automotive internal combustion engine, comprising: determining a camshaft timing advance value for a camshaft which operates cylinder intake and exhaust valves of the engine; determining a steady-state position for an electronic air throttle; determining a steady-state air/fuel ratio rate; determining transient values for electronic air throttle position and air/fuel ratio; and determining a rich air/fuel ratio suitable for purging a lean NOx trap based upon a quantity of fuel suitable for operating the engine at approximately a stoichiometric air/fuel ratio, but with insufficient air, so as to cause enrichment of the air and fuel mixture.
 2. A method according to claim 1, wherein steady-state camshaft timing advance and steady-state air/fuel ratio are based upon a sensed operating position of a manually operable accelerator, as well as upon sensed engine speed.
 3. A method according to claim 1, wherein the determination of transient electronic throttle position and transient fuel supply rate is based at least in part upon the camshaft timing advance, and upon the steady-state electronic throttle position.
 4. A method according to claim 1, wherein transient electronic throttle position is determined as a function of at least the time rate of change of the camshaft timing.
 5. A method according to claim 1, wherein transient electronic throttle position is determined as a function of at least the time rate of change of the camshaft timing and the instantaneous pressure within an air inlet manifold of the engine.
 6. A method according to claim 1, wherein transient electronic throttle position comprises a steady state value and a correction value.
 7. A method according to claim 1, wherein the transient air/fuel ratio is determined so as to track and follow the filling and emptying of the intake manifold with the result that fluctuations in output torque are minimized. 